I have two boxes of Krag ammo... a Frankford 04 box and a WRA 07 box.  Both have cupro bullets and bronze cupped primers.  Both also have some split/cracked cases.  I believe this is because of the age, coupled with incorrect (before they came to me) storage.  I have never fired one, although about ten years ago, I did disassemble a round (Frankford 04) that was not part of my boxes, and loaded the powder and bullet in a modern Remington case and fired that, which fired just fine.